Needleless injectors are devices for delivery liquid drugs through the epidermis of a patient without using a conventional hypodermic needle. The normal principle of operation is to dispense a fine jet of liquid from a drug capsule at sufficiently high pressure and velocity to pierce the skin and deposit within the underlying tissues. The better designs of injectors usually have a two-phase injection pressure profile: the first is a very fast rise time from zero to a high pressure—typically in the region of 300 bars—which is the skin-piercing phase, followed by the remaining injectate at a lower pressure, which is sufficient to keep the hoke in the skin open during the injection. The high pressure is usually developed by a gas spring or pneumatic ram, or sometimes by pyrotechnic means.
Typically, the drug capsule is a cylinder with one end open, and the other having the injection orifice. A piston is located within the bore, and the drug is contained between the orifice and piston, the orifice being sealed temporarily by a rubber plug, cap or other known means.
Drug capsules are often made from a transparent thermoplastic, but at high strain rate these materials are brittle, and a problem that can occur during the high pressure phase is that the drug capsule can burst. It is possible to make the wall of the drug capsule sufficiently thick to withstand the burst pressure, but this may result in an unacceptably large device which is more difficult to make, and more expensive. This problem is exacerbated by the presence of bubbles of air trapped within the capsule after filling. This is th ought to be because of shock waves produced by the rapid collapse and expansion of the bubbles during the transition from the first and second pressure phases. The size of the bubble has an influence—those below about 2 microlitres volume having an insignificant effect. Larger bubbles, apart from the aforementioned problem, also compromise the accuracy of filing, so that an incorrect dose might be delivered. Another problem with some drugs, such as adrenaline, is that they are sensitive to the presence of oxygen, and it is necessary to reduce the volume of trapped air to a minimum.
Increasingly, it is preferred that the capsules are pre-filled by the manufacturers on specialized filling machines: this ensures good quality control, sterility, and traceability, and it follows from the foregoing that the volume of air trapped in the injectate should be as small as possible. Equally, low cost production demands high filling rates, typically less than 1 second for 1 ml fill volume. Current filling machines for both syringes and needleless injector capsules employ vacuum to reduce the amount of air trapped, but the vacuum systems operate at around 15 to 20 mbar or higher, which means that a significant amount of air remains in the syringe or capsule before the liquid drug in introduced. It is possible to design a vacuum system which can operate at lower pressure, but these require very large reservoirs, and consequently extended pump-down times and long filling cycles. It would be possible to avoid the use of reservoirs and to connect the capsule to be evacuated directly to a vacuum pump, but the final pressure, pumping times, and overall control, would be highly unsatisfactory except in the most crude applications.